Tuftsin having threonyl-cyclo-[-N.sup..epsilon. -lysyl-prolyl-arginyl-] as its analogue comprises a fragment of a heavy chain of immunoglobulins, class IgG, of a human being (Thr-Lys-Pro-Aig). This tetrapeptide reveals a considerable stimulant effect with regard to various immunological responses in vivo and in vitro. It substantially increases phagocytic activity of leukocytes and macrophages, and activates immunogenesis of the latter, etc. (cf. Proc. Natl. Acad. Sci USA, vol. 75, No. 7, 1978; E. Tzehoval, S. Segal, Y. Stabinsky, M. Fridkin, Z. Spirer, M. Feldman "Tuftsin (an Ig-associated tetrapeptide) triggers the immunogenic function of macrophages: Application for activation of programmed cells", pp. 3400-3404).
It has been suggested that tuftsin is useful as a therapeutical remedy for the treatment of diseases associated with a lower activity of leukocytes, as well as splenectomy and certain spleen disturbances accompanied by a noticeably reduced resistance of the organism against infectious diseases. In such cases tuftsin can replace .gamma.-globulin (J. Pediat. vol. 80, No. 4, 1972, D. Constantopoulos, V. D. Najjar, J. W. Smith "Tuftsin deficiency: a new syndrome with defective phagocytosis", pp. 464-572).
A disadvantage of tuftsin as a medicated compound resides in its rapid splitting by enzymes such as carboxypeptidases thus defining its short-time effect under in vivo conditions.
The cyclic analogue of the naturally-occurring phagocytosis-stimulant peptide--threonyl-cyclo-[-N.sup..epsilon. -lysyl-prolyl-arginyl-] has not been hitherto described in the literature.